Big data platforms collect so much information about so many people that correlations emerge that allow users to be slotted into marketing categories in unexpected and often unwelcome ways. In one notorious example, the New York Times revealed that companies like Target had found that pregnant women purchased specific combinations of unscented lotion and certain supplements, allowing the creation of a “pregnancy prediction” score to allow marketing of baby-related goods to women who had not publicly revealed to anyone that they were expecting. A data broker like Axciom assigns a 13-digit code to each person and assigns them to 70 “clusters” based on traits such as age, income, race, whether you live in an urban or suburban community, and where you shop.

Increasingly, every transaction, every website viewed, and every action online generates a data trail swept into the data platforms. Most websites invite dozens of companies to track users on their site and follow them across the web. One study found that in 2013, there were 328 separate companies tracking visitors on the top fifty content Web sites. Internet providers like Comcast automatically install tracking tools on their own customers. Each site a consumer uses usually deposits a tiny bit of code on their hard drive, called a “cookie” which allows companies to track and aggregate the overall activity of consumers across the whole Internet. The placement of such online cookies on tens of thousands of website was pioneered by online advertiser DoubleClick, which was acquired by Google in 2007, and allows online advertisers to display different banner ads on any site based on the profile generated by this aggregate data. Companies often bid furiously for the right to place their banner for a particular user. And the ads can follow you around the web; for example, if you view a camera on eBay, companies will bid in an auction for your “cookie” and then display ads related to digital cameras on other web pages you visit.